You can store mileage rates in Admin > Setup > Expense > Mileage Rate Sets.
Each Mileage Rate Set can contain multiple rates (based on effective date for each rate entry). Usually each rate represents one year. It is very important to understand that each year as mileage rates change the only thing you need to do is create a new row in the schedule of rates. You do NOT need to create a new Mileage Rate set every year (and should not do this as you would also need to remap the expense type to that mileage rate set). Examples of needs for actual multiple Mileage Rate Sets (vs multiple rates within a set) include US versus Canadian travel, personal vehicle versus company vehicle, employee versus subcontractor rates. It is common, however, to have only one Mileage Rate Set per system.
Navigate to Admin > Setup > Mileage Rate Sets.
Navigate to Admin > Setup > Mileage Rate Sets > List.